How to Land on the Moon | Hacker News Reader

How to Land on the Moon

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113 pointsNaOH23 comments

When Neil Armstrong brought his Lunar Module, Eagle, in to land, he found that the site was strewn with boulders and had to very quickly find somewhere else to put down.

I thought they overshot their intended landing site, in part because Armstrong missed some visual landmarks when his attention was taken away from the window several times by a 1202 alarm. The area beyond the site was rocky (which they knew beforehand) and by the time he located and touched down on a clear spot they had < 30 seconds of fuel remaining (leading to some clenched butts in mission control).

What would have happened if they hit some boulders with the lander? Was there a real chance they could have been stuck on the moon?

The walls of the LM were 0.012 inches thick. A tad under 4x the thickness of a soda can. You can imagine why hitting anything might ruin your day.

This raised the question, for me, of just how thick is the sheet metal on a typical earth-bound automobile.

It appears that steel panels in modern cars are probably something like 0.03 or 0.04 inches thick. Aluminum panels may be 0.05" or 0.06", possibly thicker or multiple layers.

The lunar lander was lightweight and relatively fragile, designed for low gravity and low pressure differential. For example the aluminum alloy skin seems vulnerable to distortion and to sharp impacts, because it was 0.3mm, about 3-4x thickness of a modern soda can, though the frame supporting it was much stronger.

I thought the safe take off angle was very limited, but some NASA research says that even up to 70 degrees tilt could be manageable if the crew handled it correctly. https://forum.nasaspaceflight.com/index.php?topic=32246.msg1...

That doesn't mean 70 degrees would be safe for landing, though. Maybe it would topple or get damaged by rocket blowback. Apollo 15 was tilted about 10 degrees and that was not a problem.

Yes there was absolutely a chance they could have been stranded in the moon. Nixon had a speech prepared in case he had to tell the world Aldrin and Armstrong weren’t coming home.

Ive heard the boulders described as the size of volkswagens. The lunar lander was designed to handle a small amount of tilt, but no one wanted to test it and pretty much agreed that trying to land on a boulder would cause it to tip over, definitely stranding them.

Just a reminder that you can watch all of this at https://apolloinrealtime.org/

The go/no-go for landing, and last few minutes before touchdown are from T= 102:42:03.

There is some footage from mission control, but I could only see faces, no actual clenched butts ;)

I want to know more... How does one know one is in orbit? How do you work out when to fire you thrusters to leave orbit and head towards something like the moon?

Radars, star trackers and some math.

Random Google link: https://timeandnavigation.si.edu/navigating-space

Kerbal space program helped me understand a little more of the basics.

The computer aid showing the trajectories helps quite a bit and that's demonstrated quite well in KSP.

As we learned in the Hidden Figures (I did anyway) the math is Euler’s method of iteration. So a desired variable (location of touchdown!) is fixed and then the inputs iterated until a successful set of inputs are found.

you need to play Kerbal Space Program....

One of the early games I played on either my ZX81 or possibly BBC B was a lunar lander simulator. Really just a timeline and some maths and some dials. You contolled the decent with some thrusters. Burning fuel makes it lighter, which changes the thrust. Don't run out of fuel, dont land to heavy, make sure you land vertically. Good times.

Nope, that's a confabulation. https://youtu.be/jlS3fr3t4Fg

I just finished watching the national geographic on Apollo. It's really well done. The drew on a lot of local news footage which gives you a unique insight into the era.

Always love newatlas.

Don't forget bringing a golf club

I landed on the moon many times in 1975 using the HP-25 moonlander program. It achieved this in 49 lines of code and a few manualy supplied data points. Boulders were never a problem.

> When the spacecraft is about 2,000 ft (610 m) from target, it switches to the landing phase. This is when the computer hands over manual control to the Commander, who guides it in for final touchdown. Slowed to a hover, the module can be steered by tilting it like a helicopter to make any necessary corrections.

A rocket landing on (top of) a column of thrust and a helicopter suspended below a disc of thrust are not even remotely the same thing. Classic failure to reason from first principles.

Here’s a suitable analogy: it’s like the difference between having a center of mass behind the center of pressure, and a center of mass in front of the center of pressure.

For the interested layman: https://en.wikipedia.org/wiki/Longitudinal_static_stability

> Here’s a suitable analogy: it’s like the difference between having a center of mass behind the center of pressure, and a center of mass in front of the center of pressure.

This "analogy" is more abstract than the original statement and is only going to make sense to someone who more-or-less also understands that one.

How about “it’s like pulling a floating balloon by a string vs. pushing the balloon from the side and trying to keep it moving in a straight line in both cases”

Or maybe “it’s like balancing a vertically-oriented baseball bat while holding it at the top vs. balancing a broom while holding it at the bottom.”

Erm, sorry, my editing was only half baked above; I tried to change my example from a broom to a baseball bat and accidentally left both in.

The idea that a rocket sitting on top of a column of thrust is significantly different than a rocket suspended underneath a column of thrust is actually a common fallacy, known as the pendulum rocket fallacy: https://en.wikipedia.org/wiki/Pendulum_rocket_fallacy

You are correct that longitudinal stability is applicable and important for most aircraft, but that is due to angle of attack and lift forces. Neither of which apply in a vacuum.

It's actually not that different, since the disc of thrust's direction in a helicopter is bound to the helicopter's orientation itself. Thus, there's no stabilizing momentum intrinsic to the system. The pilot needs to constantly balance it manually (if there's no computer help of course), which makes a helicopter so hard to operate.

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